Indifferentiated F9 embryonal carcinoma cells were found to exhibit little, if any, calcium, phospholipid-dependent protein kinase (Ca, PL-PK) activity. Retinoic acid treatment of F9 cells causes a time-dependent increase in cytosolic Ca, PL-PK activity. The appearance of Ca, PL-PK activity seems to correlate with the onset of retinoic-induced differentiation to an endoderm cell type. Conversely, when differentiated PYS cells are treated with the phorbol ester tumor promoter 12-0-tetradecanoyl phorbol-13-acette (TPA) there is a marked decrease in cytosolic Ca, PL-PK activity. The loss in acitivity (approx. 90% decrease) occurs within minutes of TPA addition, is TPA concentration dependent and is not observed when PYS cells are treated with a biologically inactive phorbol ester analog. Concomitant with the loss in cytosolic Ca, PL-PK activity there is a significant increase in the amount of Ca, PL-PK activity associated with the plasma membrane fraction. These results indicated that the rapid and tight association of Ca, PL-PK activity with the plasma membrane may be an early event in mediating some of the effects of phorbol ester analog. Concomitant with the loss in cytosolic Ca, PL-PK activity there is a significant increase in the amount of Ca, Pl-PK activity associated with the plasma membrane fraction. These results indicated that the rapid and tight association of Ca, PL-PK activity with the plasma membrane may be an early event in mediating some of the effects of phorbol esters. In fact, these findings have been instrumental in helping to establish that Ca, PL-PK may, in fact, be the phorbol ester receptor.